Endothermic and Exothermic Reactions

 Endothermic Reactions: absorb energy

 Photosynthesis is an example of an endothermic reaction. The chlorophyll and pigments in the plant absorb the light energy from the sun's rays, as endothermic reaction is that in which energy is absorbed.


Exothermic Reactions: release energy to their surroundings
       - Chemical Equation =  reactants à products + energy 

         Explosions are examples of exothermic reactions as they release energy to their surroundings.


- Molecules are held together by chemical bonds

-         add energy to break bonds

-         give off energy to join together

- If a reaction takes more energy to break bonds than it gives off to form bonds à ENDOTHERMIC
- If a reaction takes less energy to break bonds than it gives off to form bonds à EXOTHERMIC

- Enthalpy (H) is the heat contained in a system

Energy Diagrams

-         We can chart the potential energy of the chemicals as they change from reactants to products

-         Reactants start with a certain amount of energy, energy is added to start the reaction and then energy is released as the reaction proceeds.

-         The difference in energy between reactants and products determine whether a reaction is exothermic or endothermic

EXAMPLE 1.

Example 2:

Explanation of Terms:

Energy of reactants: total potential energy of all reactants in the reaction

Energy of Products: total potential energy of all products in the reaction
Energy of the activated complex: potential energy of the "transition state" between reactants and products
Activation Energy: the energy that must be added to get the reaction to progress (reactants to activated complex)
ΔH (change in enthalpy): the change in the potential energy during the reaction. It is the:
                                   energy of the products- energy of the reactants

 Why is this reaction EXOTHERMIC?

The value of ΔH is negative which means energy is given off (shown by the diagram) making this reaction exothermic.

Why is this reaction ENDOTHERMIC?

The value of ΔH is positive which means energy is absorbed making this reaction endothermic.

Energy in the Equation

- Exothermic reactions have energy term on right hand side and a negative ΔH

- Endothermic reactions have the energy term on the left hand side and a positive ΔH

Youtube TIME!!!
http://www.youtube.com/watch?v=XgiCn1IpvzM

Here is a video explaining the differences between exothermic and endothermic reactions.

By Candace